Abstract

We observe the frequency shift, l Omega, imparted to a mm-wave beam with an orbital angular momentum of l (h) over bar per photon, when the beam,is rotated at angular frequency Omega. We show that this shift, and those found in a number of experiments on the rotation of circularly polarized beams, are special cases of the rotational frequency shift recently predicted by Bialynicki-Birula and Bialynicka-Birula. The measurement also explicitly confirms a theoretical prediction by Nienhuis.